Ever since Barry Allen used the particle accelerator to travel back in time and we saw a future in which Caitlin became Killer Frost, there has been huge anticipation for the day when Caitlin turns evil. It's been slow, but we have finally arrived at a point in Season 3 in which we know that it's coming soon. The question, of course, is: When?
Judging by the trailer and images for Episode 13 "Attack on Gorilla City," it seems that the writers of #TheFlash may have found the perfect point in time to make her evil. In some of the promo images, Team Flash is put in prison on Gorilla City. Additionally, some have speculated that Caitlin and Julian will have some romance in that episode.
It does seem unusual that Caitlyn and Julian would accompany Barry and Cisco instead of Jesse and Wally, who have control of their powers. Julian is played by Tom Felton (who starred in Planet of the Apes), so that makes sense, but Caitlyn didn't accompany Barry and Cisco on their journey to Earth-2 in Season 2.
One possible explanation for this is that the writers wanted to create a situation in which Caitlyn would be forced to use her powers in order to assist Team Flash in Gorilla City. Perhaps this time she will lose control for good and that will result in Julian losing control as well. From what we know, the Philosopher's Stone is supposed to be missing, but we were left under the impression that the stone was used to give Clive Yorkin his powers in Episode 12. So, it is possible that Julian has the stone, but it would seem implausible that he would bring it with him to Earth-2. Besides, he appears to be armed with a gun in the trailer.
There was some speculation that there might be a Grodd vs. Savitar fight at some point. Maybe Julian will have the stone with him — who knows?
What This Means Following The Events Of Gorilla City
The episode after the Gorilla City two-parter is "The Wrath of Savitar," which makes it feasible. Personally, I have my doubts that they would actually have a fight like that, but it would certainly be fun to see.
Another reason why Gorilla City might be the perfect place for Caitlyn to turn evil is because of her relationship with Grodd. This relationship was explored in both Season 1 and Season 2. With Grodd having the ability to read her mind, he could learn that she is struggling with her "killer" instincts and he could use that to his advantage in the fight against Team Flash.
Regardless of whether or not it happens in Gorilla City, this is a bitter-sweet moment. I am simultaneously eager to see Killer Frost back in action, but want to see Caitlyn remain good and pure. Losing good Caitlyn on Team Flash will be a big loss, but it has to happen at some point.